Amplified CCDs

Figure 4-100. OAUGDP experimental setup schematie A, water electrodes Hy high-voltage probes PMT, photomultiplier probe CT, eurrent transformer BIAS, parasitie current elimination tool Cy, variable capacitor OSC, oscilloscope PC, computer SIGNAL, harmonic signal generator RF AMP, radiofrequency power amplifier CCD, digital camera.

Figure 4. Simplified schematic of an optical/infrared focal plane array. The detector is a thin wafer of light sensitive material that is connected to a thin layer of solid state electronics - the connection is made either by direct deposition (CCD) or bump bonding (IR detector). The solid state electronics amplify and read out the charge produced by the incident light.

Charge transfer. In the infrared, no charge transfer is required. For an optical CCD, the charge is moved to the edge of the detector where the amplifiers are located. 

Charge amplification. Very small amounts of charge must be amplified before it can be digitized and transfered to a computer. Amplification is a noisy process. At present, CCD amphfiers exhibit lower noise than infrared amplifiers. 

Infrared arrays do not have any charge transfer, since the charge is amplified at each pixel by the silicon multiplexer. Only CCDs have charge transfer and thus this section only discusses CCD array architecture. 

Figure 19. CCD array architecture, showing the parallel and serial transfers required to move the pixel charge to the output amplifier.

The CCD MOSFET is a destructive readout device - there is only one measurement per charge packet. However, an infrared amplifier can be read ouf several times, with averaging and corresponding reduction in the effective readout noise (16 reads can reduce the noise by a factor of a/Tg or 4). In theory, multiple readout of an infrared amplifier could achieve extremely low noise, but in practice, due to other complications, the noise reduction usually reaches a limit of 4-5 improvement (achieved after 16-32 reads). 

CCD detector designers try to increase the signal-to-noise ratio of an amplifier in two ways (1) increase the responsivity, or (2) decrease the random current fluctuation between source and drain. The responsivity can be increased by decreasing the amplifier size. Decreasing the amplifier size decreases the capacitance of the MOSFET. The responsivity of a MOSFET obeys the capacitor equation which relates voltage, V, to the charge Q on capacitance C V = QIC. 

The best CCD amplifiers thaf have been produced achieve read noise of 1.5 e af 50,000 pixels per second and 4 e af 1 million pixels per second. (These are rates per readout port. Lower noise can be achieved for a given frame rate by adding more readout ports.) The besf readouf noise achieved wifh infrared defectors is 15 e for a single read and 3-5 e affer many samples. 

A modern spectrophotometer (UV/VIS, NIR, mid-IR) consists of a number of essential components source optical bench (mirror, filter, grating, Fourier transform, diode array, IRED, AOTF) sample holder detector (PDA, CCD) amplifier computer control. Important experimental parameters are the optical resolution (the minimum difference in wavelength that can be separated by the spectrometer) and the width of the light beam entering the spectrometer (the fixed entrance slit or fibre core). Modern echelle spectral analysers record simultaneously from UV to NIR. 

Many destination vectors are commercially available. However, it is also possible to construct a destination vector that is suitable for individual experiments. To constrnct snch vectors, one should insert the rr ffi-containg cassette (commercially available from Invitrogen) into the appropriate locns of a Gateway-incompatible vector. The resultant plasmids can be amplified only in the DB3.I strain, due to the toxic ccd gene (7). 

Theoretically, CCD s offered the most attractive features of the remaining choices, photodiodes and CCD s. However, at the time when the decision had to be made, CCD technology was, and still is, too much in flux, for their use in a mass spectrometer system and too higji in cost to be a reasonable choice. The cost factor would be amplified even further when one considers the increased requirements on the data acquisition system due to the 60-fold increase in data rate ( 860,00 vs 14,300 ) data points per spectrum. These and other considerations led to the decision to implement the second generation detector with a photodiode (Reticon) based camera. This system is now in operation producing excellent data and will be described in detail in the following section. 

Figure 6 Block diagram of the two-color optical parametric amplifier (OPA) and IR-Raman apparatus. CPA = Chirped pulse amplification system Fs OSC = femtosecond Ti sapphire oscillator Stretch = pulse stretcher Regen = regenerative pulse amplifier SHGYAG = intracavity frequency-doubled Q-switched Nd YAG laser YAG = diode-pumped, single longitudinal mode, Q-switched Nd YAG laser KTA = potassium titanyl arsenate crystals BBO = /J-barium borate crystal PMT = photomultiplier tube HNF = holographic notch filter IF = narrow-band interference filter CCD = charge-coupled device optical array detector. (From Ref. 96.)...

Another electro-optical ion detector, which is called the electro-optical array detector, allows the simultaneous measurement of ions spatially separated along the focal plane of the mass spectrometer. It combines the microchannel plate and Daly detector. The ions are converted in a microchannel plate into electrons that are amplified. The released secondary electrons finally strike a phosphorescent screen that emits photons. These photons are then detected with a photodiode array or CCD detector. This array detector acts as electronic photoplates. 

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